The flux method is one of the liquid phase methods. For gallium nitride, the use of sodium metal as a flux can significantly lower the temperature and pressure required for the crystal growth of gallium nitride. Specifically, nitrogen gas is dissolved in a mixed melt of sodium metal and gallium metal to bring gallium nitride into a supersaturated state, so that gallium nitride grows as crystals. In such liquid phase methods, dislocation hardly occurs as compared with the vapor phase methods, thereby high-quality gallium nitride having a low dislocation density can be obtained.
For example, in a method in which gallium nitride crystals are grown in the melt containing an alkali metal-containing flux and gallium, a crystal growth rate is slower than that in the vapor phase methods. When the high temperature/high pressure conditions are applied to increase the crystal growth rate in the flux method, a supersaturation degree of nitrogen in the flux is increased, thereby causing non-uniform nucleation (miscellaneous crystals) at the gas-liquid interface and resulting in deterioration of the crystals. Thus, it has been difficult to increase the crystal growth rate more than a certain degree.
It has been proposed that a hydrocarbon having the boiling point higher than the melting point of the alkali metal is added to the melt in order to suppress the occurrence of the miscellaneous crystals during the growth of the crystals and make uniform the thickness of the crystal film (Patent document 1: JP 4821007B). Further, the document mentions that the hydrocarbon added to the melt is preferably liquid from the standpoint of preventing the oxidation of the alkali metal by coating its surface and easiness in weighing.
Further, Patent document 2 (JP 4538596B) proposes that carbon that generates cyanide (CN) in the melt is added to the melt in order to suppress the miscellaneous crystals in the melt.
Further, the present inventors disclosed that the conductivity of crystals to be grown was controlled by using liquid dopants (Patent document 3). However, it became clear that this method was inadequate for controlling the dopant concentration in each growth batch.